In the manufacture of glass fibers, glass batches are employed which have a low alkali metal content, i.e., typically below 2 percent by weight on an Na.sub.2 O basis. In the production of pellets for glass batch feeding soda-lime glass pellets have been described extensively in the literature. Thus, glass making pellets have been described in U.S. Pat. No. 3,880,639 which employ soda-lime batch ingredients. A similar manufacture of glass making pellets is described in U.S. Pat. No. 2,366,473. In the latter patent the use of ground glass in the pelletizing of soda-lime glass is described as having an advantage because of the tendency for the soda lime ingredients to chemically set the ground glass.
A typical fiber glass batch is shown in U.S. Pat. No. 2,334,961 and that batch, like most fiber glass batches, contains minor quantities of Na.sub.2 O (2 percent or less) and relatively large quantities of B.sub.2 O.sub.3, i.e., above 6 percent by weight, typically 8 to 12 percent by weight.
Boron-containing fiber glass batch pellets utilizing boric acid, anhydrous boric acid or colemanite as the boron source may be prepared by introducing the batch ingredients in appropriate proportions into a pelletizing zone, such as an inclined rotating disc pelletizer, as is shown in U.S. Pat. No. 3,914,364, which is incorporated herein by reference, and adding to the batch ingredients as they are rotated on the pelletizer sufficient water to agglomerate the batch materials and support the continuous production of glass batch pellets of a desired size. In lieu of a disc pelletizer, rotary kilns or other similar devices may be employed. The pellets range in nominal diameter, for example, from about 0.125 to about 1.00 inch (0.3175 to 2.54 centimeters) and preferably between about 0.375 and 0.625 inch (0.9525 and 1.5875 centimeters). Sufficient water is added to bind the batch ingredients together and provide pellets of the batch materials.
Preferably, these pellets contain approximately 5 to 22 percent by weight free water, when boric acid is employed, 10 to 25 percent free water when anhydrous boric acid is employed and 5 to 20 percent free water when colemanite is employed. Most preferably, the water is added to provide approximately 11 to 13 percent by weight free water for boric acid, 15 to 17 percent free water for anhydrous boric acid and 10 to 13 percent free water for colemanite. The pellets after formation are dried at temperatures preferably from about 220.degree. F. (104.4.degree. C.) or less up to about 1000.degree. F. (537.8.degree. C.) or more for boric acid, at a temperature in excess of 450.degree. F. (232.2.degree. C.) for anhydrous boric acid and at temperatures preferably between about 220.degree. F. (104.4.degree. C.) or less to 770.degree. F. (410.degree. C.) for colemanite for a sufficient period of time to provide a free water conent preferably of below about 1 percent by weight. Hard, substantially non-dusting pellets are thus produced. The hard, non-dusting pellets thus formed can be fed to a glass melting furnace and exposed to conditions in excess of 2700.degree. F. (1482.2.degree. C.) without any explosions of the pellets occurring.
When colemanite is employed as a boron source, the batch ingredients may contain only colemanite as the boron source. Optionally, up to about 75 percent or more of the colemanite, on a B.sub.2 O.sub.3 basis, may be substituted by boric acid, while adjusting for lost silica, calcium and aluminum contained in the colemanite.
In a typical "E" glass type boron-containing glass fiber forming batch composition, such as is illustrated in U.S. Pat. No. 2,334,961, silica, clay, limestone, coal, fluorspar, sodium sulfate, ammonium sulfate and boric acid are used as the ingredients. In lieu of boric acid, colemanite may be used. The use of colemanite is described in U.S. Pat. No. 3,274,006. Colemanite has a chemical composition of Ca.sub.2 B.sub.6 O.sub.11 Ca.sub.2 B.sub.6 O.sub.11 . 5 H.sub.2 O. Optionally, boric acid may be substituted for up to about 75 percent or more of the colemanite on an equivalent B.sub.2 O.sub.3 basiswhile adjusting for lost silica, calcia and alumina from the colemanite.
These glass batch ingredients, when used to prepare pellets, are believed to undergo several chemical reactions during their deposition on the pelletizing disc and while water in the quantity sufficient to produce the pellets is being added to the ingredients and during the drying of the pellets. The primary reactions involved in the preparation of the pellets are believed to be as follows: ##EQU1##
In reaction (1) the anhydrous boric acid, if employed, is reacted with water to form boric acid. In reaction (2) the boric acid and water react to dissolve the boric acid and then the boric acid is recrystallized as shown in the equation. The recrystallized boric acid is dehydrated during the drying step to drive water off, as can be seen in equation (3). Some of the boric acid itself, during the drying of the pellets, reacts with the calcium carbonate present to form hydrated calcium pyroborate, carbon dioxide, and water in accordance with equation (4). Boric acid also reacts with the sodium sulfate present in the batch in accordance with equation (5) to form hydrated sodium tetraborate and sulfuric acid. Limestone and sulfuric acid may also react to form calcium sulfate, carbon dioxide and water, in accordance with equation (6).
It is extremely important in the preparation of glass batch pellets that if colemanite is used as the single source of B.sub.2 O.sub.3, temperatures above about 770.degree. F. (410.degree. C.) by avoided during the drying step. Attempts to dry these pellets above this temperature result in the disintegration of the pellets and their return to the powdery state. Thus, extreme care is taken to provide pellets by regulating the drying operation, such that the pellets are dried at temperatures not exceeding 770.degree. F. (410.degree. C.).
When boric acid is substituted for up to about 75 percent or more of the colemanite, with proper adjustments being made for silica, calcium and alumina in composition, these temperature parameters must still be followed. Hard, non-dusting pellets can be produced at drying temperatures up to 770.degree. F. (410.degree. C.).
When drying the colemanite containing pellets, should the temperature of the pellets exceed 770.degree. F. (410.degree. C.), it has been found that the pellets crack and disintegrate. This problem, however, may be solved by pretreating the colemanite prior to its addition to the glass batch, and then forming the glass batch using the pretreated colemanite into pellets. This pretreatment comprises heating the colemanite at a temperature above 770.degree. F. (410.degree. C.) for a sufficient period of time until substantially all of the chemically bound water in the colemanite is driven from this material. An equation for this reaction is shown above as equation (7). This water amounts to approximately 21 to 22 percent by weight. There is also an expansion of up to 33 percent by volume of the colemanite when the chemically bound water is driven off, which accounts for the cracking of pellets when heated above this temperature using untreated colemanite. By employing this pretreatment to the colemanite prior to its introduction into the glass batch, pellets as heretofore described may be dried at any temperature from about 220.degree. F. or less (104.4.degree. C.) to the melting point of a given pellet, and preferably between about 220.degree. F. and 1000.degree. F. (104.4.degree. C. and 537.8.degree. C.), without fear of cracking, to produce hard, non-dusting pellets.
The preparation of the fiber glass batch pellets above referred to are described in more detail in my co-pending applications Ser. No. 739,883, Ser. No. 739,884 and Ser. No. 739,885, each of which was filed on Nov. 8, 1976, now U.S. Pat. Nos. 4,074,989, 4,074,990 and 4,074,991, respectively, and all three of which are incorporated herein by reference.